Encased semiconductor laser device in contact with a fluid and method of producing the laser device

ABSTRACT

At least part of the waveguide of a laser, the waveguide including a first cladding layer, an active layer, and a second cladding layer of a second conductivity type, and, for a ridge type laser, a ridge in the second cladding layer, has a width such that light leaks from the side walls of the waveguide. A case encloses the side walls of the waveguide and a fluid having a refractive index is sealed in the case in contact with the side walls of the waveguide. A characteristic of the laser can be adjusted easily. Therefore a laser having a uniform characteristic can be provided at a low cost. This laser is useful as a light source for wavelength multiplex transmission used for optical transmission, of a main line system, such as a submarine cable.

TECHNICAL FIELD

The present invention relates to a laser device and a method ofproducing a semiconductor device and, more particularly, to a laserdevice that allows adjustment of characteristic values of asemiconductor laser element after producing the laser, and a method ofproducing the laser device.

BACKGROUND ART

As the society becomes increasingly dependent on the informationtechnologies, various communications apparatuses are being imposed withrequirements to have multimedia capabilities. Thus processing speeds anddata handling capacities of these apparatuses have been increased, andattempts have been made for the application of wavelength divisionmultiplexing transmission technology to optical signal transmission overtrunk lines such as submarine cables.

In the wavelength division multiplexing transmission, distributedfeedback semiconductor laser devices (hereafter referred to as DFB-LD)is used as the light source with 20 to 100 DFB-LDs of differentoscillation wavelengths being arranged in an array, each oscillating toemit light at a predetermined wavelength that is transmitted by anoptical cable and combined with other light in a coupler. It istherefore important in achieving wavelength division multiplexingtransmission to stabilize the oscillation wavelength of each DFB-LD andaccurately determine the intervals between the oscillation wavelengths.

A band of wavelengths used in the wavelength division multiplexingtransmission is defined according to a recommendation by ITU-T(International Telecommunications Union, TelecommunicationStandardization Sector), with the interval between adjacent wavelengthsbeing regulated to be 0.8 nm. Accordingly, it is recognized that theDFB-LD used as the light source preferably oscillates at a wavelengthwithin ±0.1 nm of the defined wavelength.

However, the DFB-LD of the prior art has a problem that, since theaccuracy of oscillation wavelength has been limited to about 1 nmbecause the processing margin is insufficient when producing an elementwhile setting a particular wavelength in the processes of crystal growthand wafer processing, use of the DFB-LD as the light source forwavelength division multiplexing transmission has been impractical forthe reason of the accuracy of oscillation wavelength.

Sudoh et al. (ELECTRONICS LETTERS; Jan. 30, 1997 Vol.33, No.3,p.216-p.217) have recently reported a DFB-LD that enabled it to tune theoscillation wavelength to a desired value by a method as follows: A partof an optical waveguide of a DFB-LD is provided with a film made of amaterial that changes refractive index depending on the heat generatedby laser irradiation, and the refractive index of the film is changed byirradiating it with a laser beam while measuring the oscillationwavelength of the DFB-LD after the laser element has been made, therebychanging the effective refractive index of the waveguide and achievingthe desired oscillation wavelength.

Although demands for laser elements having more accurate oscillationwavelengths are increasing, the demands cannot be satisfied in theproduction process only. Thus such techniques have been developed thatachieve laser elements having the desired oscillation wavelengths byadjusting the oscillation wavelength after producing the laser element.

Adjustment of the characteristic values to be done after producing thelaser element includes, in addition to the oscillation wavelength of theDFB-LD, adjustment of other characteristic values such as the photondensity in an active layer of a ridge type semiconductor laser.

FIG. 10 is a sectional view showing the structure of a DFB-LD of theprior art.

In FIG. 10, reference numeral 1 denotes an n-InP substrate, 2 denotes ann-InP buffer layer, 3 denotes an n-InGaAsP light confinement layer, 4denotes an MQW active layer, 5 denotes a p-InGaAsP light confinementlayer, 6 denotes a diffraction grating layer, 7 denotes a p-InP firstcladding layer, 8 denotes an Fe-doped InP embedding layer, 9 denotes ann-InP embedding layer, 10 denotes a p-InP second cladding layer, 11denotes a p-InGaAs contact layer, 12 denotes an SiO2 insulating film,.13denotes a Cr/Au vapor deposited film and 14 denotes an anode ofAu-plating layer. 15 denotes a metallic vapor deposited film and 16denotes a cathode of Au-plating layer provided on the surface of themetallic vapor deposited film.

Oscillation wavelength λ of the LD having the structure described aboveis given as follows assuming the effective refractive index neff of theoptical waveguide and the interval Λ of the diffraction grating.

λ=2·neff·Λ

When Λ is 240 nm and neff is 3.23, for example, then λ is 1550.4 nm.

Factors that determine the value of neff include the distribution ofrefractive index of the material in a region from which light leaks outthat is a circular area about 2 μm in diameter, and particularlyimportant are composition and film thickness of the n-InGaAsP lightconfinement layer 3, the MQW active layer 4 and the p-InGaAs lightconfinement layer 5 that constitute the optical waveguide and the widthof the optical waveguide.

However, the DFB-LD shown in FIG. 10 is difficult to produce due tovariations in the process, while maintaining uniform conditions for thecomposition and film thickness of the n-InGaAsP light confinement layer3, the MQW active layer 4 and the p-InGaAs light confinement layer 5that constitute the optical waveguide and the width of the opticalwaveguide that are the factors which determine the value of neff, andtherefore it has been difficult to produce the DFB-LD having oscillationwavelength of accuracy within ±0.1 nm due to the variation in neff.

One of the solutions for this problem is the wavelength tuning DFB-LDstructure proposed by Sudoh et al. described previously.

FIG. 11 is a sectional view showing the structure of the wavelengthtuning DFB-LD of the prior art.

In FIG. 11, reference numeral 21 denotes an n⁺-InP substrate, 22 denotesan n-InP layer, 23 denotes an active layer, 24 denotes a diffractiongrating layer, 25 denotes a p-InP layer, 26 denotes a p⁺-InGaAs layer,27 denotes Ti/Au electrode, 28 denotes a wavelength control film made ofAs₄Se₅Ge₁ and 29 denotes an Al₂O₃ film.

When the wavelength control film 28 was irradiated with light emitted bya He-Ne laser of wavelength 632.8 nm with power density of 1.3 W/cm², awavelength shift of 0.14 nm was observed.

FIG. 12 shows an embedding type DFB-LD of wavelength tuning type of theprior art which is a modification of the wavelength tuning DFB-LDstructure proposed by Sudoh et al. turned into an embedding type DFB-LDshown in FIG. 10.

In FIG. 12, reference numerals identical with those used in FIG. 10 andFIG. 11 denote the same or corresponding components.

Changes in the refractive index of As₄Se₅Ge₁ that constitutes thewavelength control film 28 due to the wavelength of Ar laser light arereported in the ELECTRONICS LETTERS mentioned previously by Sudoh et al,indicating that maximum change in the refractive index at wavelength1.55 μm was 0.027.

Thus assuming that width w of the optical waveguide of FIG. 11 is 1.3μm, total width W of the optical waveguide including the embeddinglayers is 1.7 μm and thickness of the wavelength control film 28 is 0.5μm, then the range of neff values of the adjustable effective refractiveindex determined upon computation of the light propagation mode is from3.18716 to 3.18728. Consequently, when Λ is 240 nm, adjustable range ofthe oscillation wavelengths is from 1529.84 nm to 1529.89 nm, giving atunable band of 0.05 nm.

However, when the wavelength control film made of As₄Se₅Ge₁ is used asin the method described above, since the change in the refractive indexof As₄Se₅Ge₁ caused by laser irradiation is an irreversible change,refractive index of the wavelength control film made of AS₄Se₅Ge₁ whichhas once decreased cannot be increased. Therefore, a failure in tuningthe wavelength results in a rejected product out of the wavelengthstandard which cannot be tuned again. Thus there has been such a problemthat the tuning operation must be done very carefully, leading toincreased time taken which results in an increased production cost ofthe semiconductor laser element.

The present invention has been made to solve the problems describedabove.

A first object of the present invention is to provide a laser devicethat allows it to easily adjust the characteristic values of a laserelement, thereby to produce the laser devices having uniformcharacteristics easily at a low cost.

A second object of the present invention is to provide a distributedfeedback semiconductor laser device that allows it to easily adjust theoscillation wavelength and has a predetermined oscillation wavelength ofhigh accuracy.

A third object of the present invention is to provide a ridge typesemiconductor laser that allows it to adjust the photon density in anactive layer and has a low threshold of oscillation at a low cost.

A fourth object of the present invention is to provide a method ofproducing a laser device that allows it to easily adjust characteristicvalues of a laser element, thereby to obtain the laser devices havinggood characteristics at a low cost.

Prior art for producing laser devices include, besides that shown inFIG. 10 through FIG. 12, for example, a semiconductor laser devicedisclosed in Japanese Patent Kokai Publication No. 8-116138. It is asemiconductor laser device having a semiconductor laser element sealedin a case that has a laser light emission window, while the element iscooled by circulating a liquid which is transparent to the light of theoscillation wavelength of the laser. The application mentioned above hasno description with regards to forming at least a part of layers of thewaveguide of the semiconductor laser element to such a predeterminedwidth as light leaks from a side wall of the waveguide, and toadjustment of the oscillation wavelength by means of the fluid thatmakes contact with this part.

DISCLOSURE OF THE INVENTION

The laser device according to the present invention comprises asemiconductor substrate, a semiconductor laser element made in such aconfiguration as at least a part of layers of a waveguide that includesa first cladding layer of a first conductivity type, an active layer anda second cladding layer of a second conductivity type disposedsuccessively on the substrate is formed to a predetermined width thatallows light to leak from a side wall thereof along the direction oflight propagation, a case that houses the semiconductor laser elementdisposed therein, the case surrounding the side wall of the waveguideand having an aperture capable of letting the fluid flow in and thensealing the case, and a fluid that is sealed in the case to make contactwith the side wall and has a predetermined refractive index, whereinintensity of light leaking from the side wall can be regulated bychanging the refractive index of the fluid contacting the side wall ofthe waveguide, thus making it possible to adjust the characteristicsvalues of the laser element.

According to the present invention, the waveguide is further providedwith a diffraction grating layer, so that the effective refractive indexof the optical waveguide can be controlled and the oscillationwavelength of the DFB-LD can be adjusted by changing the refractiveindex of the fluid that makes contact with the side wall of thewaveguide.

Also a part of thickness of the second cladding layer of the waveguideis formed into a ridge that has a predetermined width which allows lightto leak, so that the photon density in the active layer of the ridgetype laser can be adjusted by regulating the intensity of light leakingfrom the second cladding layer.

Also the laser element is disposed in the case that surrounds the sidewall and has a first part comprising the aperture capable of letting thefluid flow in and then sealing the case and a second part having anemission window provided therein to oppose an light emitting end face ofthe laser element, the first part and the second part being disposed tointerpose a partition wall that seals off the fluid, so that there willoccur no decrease in the emission efficiency and in the single-modeoscillation performance since the laser light emission end face does notmake contact with the fluid that has the predetermined refractive indexand the reflectivity of the emission end face does not change.

Moreover, the laser element is disposed in the case and walls thatconstitute a same chamber of the case have the emission window disposedto oppose the light emitting end face of the laser element and theaperture capable of letting the fluid flow in and then sealing the case,thereby making the constitution simple.

Furthermore, silicon oil is used as the fluid that has high heatconductivity, and therefore thermal stability of the laser device isimproved.

The method of producing the semiconductor device according to thepresent invention includes a process of operating the semiconductorlaser element, letting fluids of different refractive indicessuccessively through the aperture into the case, and adjusting thecharacteristic values while measuring the characteristic values of thelaser element, and therefore the characteristic values of the laserelement can be adjusted in a reversible manner, thus resulting inimproved yield of production.

Also because an inlet aperture and an outlet aperture are provided sothat fluids of different refractive indices can be caused tocontinuously flow in and out through the inlet aperture and the outletaperture while adjusting the characteristic values, thus time taken inthe adjustment can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partially transparent perspective view of a laser deviceaccording to the first embodiment of the present invention.

FIG. 2 is a sectional view of a laser device according to the firstembodiment of the present invention.

FIG. 3 is partially enlarged sectional view of a laser element accordingto the first embodiment of the present invention.

FIG. 4 is a sectional view of the laser element according to the firstembodiment of the present invention.

FIG. 5 is a graph showing the relation between the refractive index ofsilicon oil and oscillation wavelength of DFB-LD according to thepresent invention.

FIG. 6 is a sectional view of a laser device according to the secondembodiment of the present invention.

FIG. 7 is a perspective view of a laser device according to the thirdembodiment of the present invention.

FIG. 8 is a sectional view of the laser device according to the thirdembodiment of the present invention.

FIG. 9 is a perspective view of a variation of the laser elementaccording to the third embodiment of the present invention.

FIG. 10 is a sectional view of a prior art laser device.

FIG. 11 is a sectional view of a prior art laser device.

FIG. 12 is a sectional view of a prior art laser device.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiment of the present invention provides a laser devicecomprising a semiconductor substrate, a semiconductor laser element madein such a configuration as at least a part of layers of a waveguide thatincludes a first cladding layer of a first conductivity type, an activelayer and a second cladding layer of a second conductivity type disposedsuccessively on the substrate, for example the waveguide that includesthe first cladding layer, the active layer and the second cladding layerof the second conductivity type in the case of the DFB-LD, or a ridgeformed on a residual thickness of the second cladding layer in the caseof a ridge type laser, is formed to a predetermined width that allowslight to leak from a side wall along the direction of lighttransmission, a case that houses the semiconductor laser elementdisposed therein while surrounding the side wall of the waveguide andhaving an aperture capable of letting a fluid flow in and then sealingthe case, and a fluid that is sealed in the case to make contact withthe side wall and has a predetermined refractive index.

According to a method of producing the laser device, for thesemiconductor laser element made in such a configuration as at least apart of layers of waveguide that includes a first cladding layer of afirst conductivity type, an active layer and a second cladding layer ofa second conductivity type disposed successively on the substrate, forexample the waveguide that includes the first cladding layer, the activelayer and the second cladding layer of the second conductivity type inthe case of the DFB-LD, or a ridge formed on a residual thickness of thesecond cladding layer in the case of a ridge type laser, is formed to apredetermined width that allows light to leak from a side wall along thedirection of light transmission, a case that houses the semiconductorlaser element disposed therein while surrounding the side wall of thewaveguide and having an aperture capable of letting the fluid flow inand then sealing the case, wherein the semiconductor laser element isoperated, fluids of different refractive indices are circulatedsuccessively through the aperture into and out of the case, and thecharacteristic values are adjusted while measuring the characteristicvalues of the laser element.

Accordingly, it is made possible to constitute the laser device thatallows it to easily adjust the characteristic values of the laserelement, thereby to produce the laser devices having uniformcharacteristics at a low cost.

Also according to the method of producing the laser device describedabove, it is made possible to easily adjust the characteristic values ofthe laser element to predetermined values, and produce the laser deviceshaving uniform characteristics at a low cost

Now particular embodiments of the invention will be described below.

A. Embodiment 1

FIG. 1 is a partially transparent perspective view of a laser deviceaccording to the first embodiment of the present invention.

In FIG. 1, reference numeral 30 denotes a laser device, 31 denotes apackage serving as a case made of, for example, iron, copper or acopper-tungsten alloy. Reference numeral 32 denotes a fluid inlet portprovided on a wall of the package, 33 denotes a fluid outlet portprovided on a wall of the package 31, both the inlet port 32 and theoutlet port 33 being hermetically sealed with plugs 36 after the insidehas been filled with the fluid. 34 denotes a laser light emission windowmade of a material that is transparent to the laser light, such asquartz. An arrow drawn on the emission window indicates the laser light.35 denotes an anode terminal.

FIG. 2 is a sectional view of the laser device shown in FIG. 1 in across section perpendicular to the laser light emitting direction. FIG.3 is an enlarged sectional view of portion A enclosed in a circle inFIG. 2. FIG. 4 is a sectional view of the semiconductor laser element ina cross section (along line indicated by arrows in FIG. 2) parallel tothe laser light emitting direction.

In FIG. 2, FIG. 3 and FIG. 4, reference numeral 40 denotes an embeddingtype DFB-LD element. 41 denotes an n-InP substrate containing n-typeimpurity of Sn or S with impurity concentration of 1×10¹⁸ cm⁻³(hereafter denoted as 1E18 cm⁻³). 42 denotes an n-InP buffer layer withimpurity concentration in a range from 1E18 cm⁻³ to 1E19 cm⁻³, 43denotes an n-InGaAsP light confinement layer with impurity concentrationof 5E17 cm⁻³, 44 denotes an MQW active layer formed by stacking InGaAsand InGaAsP alternately and 45 denotes a p-InGaAsP light confinementlayer containing p-type impurity of Zn with impurity concentration of5E17 cm⁻³, 46 denotes a diffraction grating formed by etching an InGaAsPlayer, 47 denotes a p-InP first cladding layer with impurityconcentration in a range from 5E17 cm⁻³ to 1E18 cm⁻³, 48 denotes anFe-doped InP embedding layer, 49 denotes an n-InP embedding layer withimpurity concentration of 1E18 cm⁻³, 50 denotes a p-InP second claddinglayer with impurity concentration in a range from 5E17 cm⁻³ to 1E18cm⁻³, 51 denotes a p-InGaAs contact layer with impurity concentration of1E19 cm⁻³, 52 denotes an SiO₂ insulating film, 53 denotes a Cr/Auvapor-deposited film and 54 denotes an Au-plating layer with the Cr/Auvapor-deposited film 53 and the Au-plating layer 54 constituting ananode. 55 denotes a vapor-deposited film comprisingAuGe/Ni/Ti/Pt/Ti/Pt/Au, 56 denotes an Au-plating layer formed on thesurface of the vapor-deposited film 55, while the vapor-deposited film55 and the Au-plating layer 56 constituting a cathode. 57 denotes afluid that has a predetermined refractive index that fills the package.Silicon oil is used for the fluid. Use of silicon oil as the fluid makesit possible to adjust the refractive index and makes it easy to cooldown the laser element due to the high heat conductivity.

The Au-plating layer 54 is connected by a lead wire 58 to the anodeterminal 35. The Au-plating layer 56 is provided on the package 31, andis grounded via the package 31. In order to remove heat generated whenthe DFB-LD element 40 is operating, the package 31 is placed on acooling device (not shown) that employs a Peltier element, thereby tocool the fluid 57 that fills the package 31 and prevents the DFB-LDelement 40 from being heated to a high temperature.

Now the operation of the DFB-LD will be described below.

When positive holes are injected through the anode and electrons areinjected through the cathode so that electrons and positive holes areconfined in the MQW active layer 44 with the electron-hole densityreaching a sufficient level, stimulated emission of light takes placethus achieving light amplification. A resonator is formed by providingthe diffraction grating 46 in an amplification region where the lightoutput is amplified, with at least a part of the light output from theamplification region is returned to this region by using the diffractiongrating 46, thereby forming a feedback loop and maintaining oscillation,with laser light being emitted through the emission end face.

The diffraction grating 46 has a portion where the phase is shifted byλ/4 (λ represents the wavelength) in the middle of the LD resonator. TheMQW active layer 44 is interrupted near the front end and rear end ofthe LD, where windows 63, 64 are formed by embedding with the Fe-dopedInP embedding layer 48, the n-InP embedding layer 49 and the p-InPsecond cladding layer 50. Formed on the front end face and the rear endface of the LD are vapor-deposited films made of Al₂O₃ with a thicknessof λ/4 not shown in the drawing, thus reducing the reflectivity.

Now the method of producing the laser device 30 will be described below.

The DFB-LD element 40 used in the laser device 30 is made by forming then-InP buffer layer 42, the n-InGaAsP light confinement layer 43, the MQWactive layer 44, the p-InGaAsP light confinement layer 45, the p-InPfirst cladding layer 47 and the diffraction grating layer 46successively on the n-InP substrate 41 by vapor phase growth process, adiffraction grating is formed by etching, and then the p-InP firstcladding layer 47 is formed again, thereby embedding the diffractiongrating. Then the layers are etched until the n-InP substrate 41 isexposed, leaving a ridge 60 a having width w of 1.3 μm as an opticalwaveguide 60.

The ridge 60 a is then embedded by forming the Fe-doped InP embeddinglayer 48 and the n-InP embedding layer 49 by selective growth on bothsides of the ridge 60 a, followed by the formation of the p-InP secondcladding layer 50 and the p-InGaAs contact layer 51 on the ridge 60 aand the n-InP embedding layer 49.

Then the layers on both sides of the ridge 60 a that is located at thecenter are etched until the n-InP substrate 41 is exposed, leaving aportion of width not greater than 2 μm, thereby to form a semiconductormesa, followed by the formation of the SiO₂ insulating film 52 on thesurface and removal of that over the ridge 60 a. Then the Cr/Auvapor-deposited film 53 and the Au-plating layer 54 are formed therebyto form the anode.

Assuming the semiconductor mesa 61 that includes the optical waveguide60, the embedding layers 48, 49 formed on both sides of the opticalwaveguide 60 and the SiO₂ insulating film 52 has width W, since theregion where light leaks is a circular region of radius about 2 μm, itis necessary to keep the width W within about 4 μm in order to controlthe oscillation wavelength of the laser light by regulating theintensity of the leaking light, and the smaller the width, the easier tocontrol the wavelength. However, the width cannot be made too small dueto the necessity to form the anode. Thus width W is set to be greaterthan the width of the ridge 60 a and not greater than 4.0 μm. Morepreferably, width W is set to be greater than the width of the ridge 60a and not greater than 1.2 μm.

The DFB-LD element 40 formed as described above is disposed in thepackage 31 which is then filled with the silicon oil 57 of whichrefractive index is controlled to a predetermined value, and the sidewalls of the semiconductor mesa 61 is immersed in the silicon oil 57.

The refractive index of the silicon oil 57 corresponding to theoscillation wavelength required is determined as described below.

Silicon oils of different refractive indices are prepared as D. Bosc etal described in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol.9, No.5, May 1977p648-p650. That is, by using a mixture of a plurality of silicon oilshaving different kinds of impurity in different concentrations, it ismade possible to change the refractive index of a cavity 62 filled withthe silicon oil in the package 31 in a range from 1.432 to 1.447.

Then the silicon oils of different refractive indices are poured intothe package 31 so that the side walls of the semiconductor mesa 61 isimmersed in the silicon oil 57. At this time, the DFB-LD is operated andthe silicon oils of different refractive indices are poured into thepackage successively while monitoring the oscillation wavelength with anoptical spectrum analyzer. In order to change the refractive index ofthe silicon oil continuously, a mixture of two kinds of silicon oils ofdifferent refractive indices may be poured into the package 31 whilechanging the mixing proportion continuously. This procedure can reducethe time taken in the adjustment.

When the predetermined oscillation wavelength is obtained, supply of thesilicon oil is stopped and the inlet aperture 32 and the outlet aperture33 are sealed with the plugs 36, thereby fixing the oscillationwavelength of the laser device 30.

With the procedure of adjusting the wavelength using the silicon oils ofdifferent refractive indices, when the refractive index of the siliconoil mixture is set too high or too low, the refractive index of thesilicon oil mixture can be corrected in the reverse direction. Namely,oscillation wavelength can be adjusted reversibly.

Assuming that ridge width w of the optical waveguide 60 is 0.7 μm andtotal width W of the semiconductor mesa 61 including the opticalwaveguide is 1.0 μm, for example, then the range of effective refractiveindex neff of the waveguide 60 changes in a range from 3.1486 to 3.14190when the refractive index of the cavity 62 filled with the silicon oilin the package 31 is changed from 1.432 to 1.447. Consequently, whenassuming Λ is 240 nm for the diffraction grating, adjustable range ofthe oscillation wavelength of the laser device 30 is from 1508.093 nm to1508.11 nm.

FIG. 5 is a graph showing the relation between the refractive index ofsilicon oil and the oscillation wavelength of the DFB-LD according tothe present invention, based on the result described above.

Refractive indices of the materials used near the waveguide 60 are 3.3for the n-InGaAsP light confinement layer 43 and the p-InGaAsP lightconfinement layer 45, from 3.4 to 3.6 for the MQW active layer 44, 3.3for the diffraction grating 46, 3.17 for the p-InP first cladding layer,and 3.17 for the Fe-doped InP embedding layer, the n-InP embedding layer49 and the p-InP second cladding layer 50.

In this embodiment, since the fluid 57 used for the control of theoscillation wavelength has a low refractive index of about 1.44, lessintensity of light leaks into the portion of the liquid 57 capable ofadjusting the refractive index, thus resulting in smaller range ofoscillation wavelengths adjustable. However, provided that a materialhaving higher refractive index can be used for the liquid 57, intensityof light leaking into the liquid 57 can be increased, thereby increasingthe adjustable range of the oscillation wavelengths. Although thesilicon oil is used for the liquid to control the refractive index inthis embodiment, the liquid is not limited to a particular material andany materials that have different refractive indices and can be mixed ina desired proportion can be used. Also instead of liquid, the refractiveindex can be changed by mixing gases having different refractiveindices.

According to this embodiment, as described above, oscillation wavelengthcan be continuously and easily changed by making the semiconductor mesa61 in such a width as light leaks out therefrom and bringing the siliconoils of different refractive indices successively into contact with theside walls of the semiconductor mesa-61. Thus the DFB-LD having theoscillation wavelength adjusted with high accuracy can be made at a lowcost, as the light source for wavelength division multiplexing opticaltransmission.

B. Embodiment 2

FIG. 6 is a sectional view of the laser device according to the secondembodiment of the present invention.

In FIG. 6, the same reference numerals as those in the first embodimentdenote the same or corresponding parts.

Reference numeral 70 denotes a ridge type laser diode element (hereafterreferred to as ridge type LD element), 71 denotes an n-GaAs substratecontaining n-type impurity of Si or Se with impurity concentration of1E18 cm⁻³, 72 denotes an n-AlGaAs cladding layer containing n-typeimpurity of Se with impurity concentration of 2E17 cm⁻³, 73 denotes ann-AlGaAs light confinement layer containing n-type impurity of Se withimpurity concentration of 2E17 cm⁻³, 74 denotes an undoped AlGaAs activelayer, 75 denotes a p-AlGaAs light confinement layer containing p-typeimpurity of Zn with impurity concentration of 5E17 cm⁻³, 76 a denotes aresidual layer of the p-type AlGaAs cladding layer containing p-typeimpurity of Zn with impurity concentration of 5E17 cm⁻³, 76 b denotes aridge portion of the p-AlGaAs cladding layer containing p-type impurityof Zn with impurity concentration of 5E17 cm⁻³, 77 denotes a p-typeAlGaAs etching stopper layer containing p-type impurity of Zn withimpurity concentration of 5E17 cm⁻³, 78 denotes a p-GaAs cap layercontaining p-type impurity of Zn with impurity concentration in a rangefrom 5E17 cm⁻³ to 1E18 cm⁻³, 79 denotes a Ti/Mo/Ti/Au vapor-depositedfilm, 80 denotes an Au-plating layer, the vapor-deposited film 79 andthe Au-plating layer 80 constituting the anode. Reference numeral 81denotes an vapor-deposited film comprising AuGe/Ni/Ti/Pt/Ti/Pt/Au, 82denotes an Au-plating layer, the vapor-deposited film 81 and theAu-plating layer 82 constituting the cathode.

This embodiment is the same as the first embodiment in the constitutionof the package 31, use of silicon oil as the fluid, connection of theanode 80 by the lead wire 58 to the anode terminal 35, mounting of thecathode 82 on the package 31, grounding of the cathode 82 through thepackage 31, and removal of heat generated during the operation of theridge type LD element 70 by placing the package 31 on a cooling device(not shown) that employs a Peltier element thereby to cool the siliconoil 57 that fills the package 31 thereby preventing the ridge type LDelement 70 from being heated to a high temperature.

Now the method of producing the ridge type LD element 40 will bedescribed below.

The n-AlGaAs cladding layer 72, the n-AlGaAs light confinement layer 73,AlGaAs active layer 74, the p-AlGaAs light confinement layer 75, theresidual layer 76 a of the p-type AlGaAs cladding layer, the etchingstopper layer 77, the p-AlGaAs cladding layer that becomes the ridgeportion 76 b and the p-GaAs cap layer 78 are formed successively byvapor phase growth process on the n-GaAs substrate 71.

Then the ridge portion 76 b is formed by etching the layers until theetching stopper layer 77 is exposed while leaving a predetermined ridgewidth from the p-GaAs cap layer 78.

The Ti/Mo/Ti/Au vapor-deposited film 79 and the Au-plating layer 80 arethen formed on the p-GaAs cap layer 78 to form the anode, while thevapor-deposited film comprising AuGe/Ni/Ti/Pt/Ti/Pt/Au 81 and theAu-plating layer 82 are formed on the back surface of the n-GaAssubstrate 71 to form the cathode.

The ridge type LD element 70 requires it to control the photon densityin the active layer 74, that makes it necessary to make the thickness dof the residual layer 76 a of the p-AlGaAs cladding layer and the ridgewidth W of the ridge portion 76 b of the p-AlGaAs cladding layer with anaccuracy of within 0.1 μm.

In case the ridge width W of the ridge portion 76 b becomes greater thanthe predetermined width in comparison to the thickness d of the residuallayer 76 a, photon density in the active layer 74 becomes higher andoptical damage of the laser may be induced. On the other hand, when theridge width W of the ridge portion 76 b becomes smaller than thepredetermined width in comparison to the thickness d of the residuallayer 76 a, photon density decreases because insufficient light isconfined in the active layer 74 located directly below the ridge portion76 b, resulting in less light confined in the region where current isinjected. This leads to lower gain of the laser and higher threshold ofoscillation.

The thickness d of the residual layer 76 a can be made accurately byproviding the etching stopper layer 77 thereby controlling the depth ofetching, although it has not been possible to make the ridge width W ofthe ridge portion 76 b uniformly with high accuracy due to side etching,and the ridge type LD element 70 has not been produced with a highyield.

In this embodiment, the laser device 31 wherein the photon densitydistribution in the active layer can be adjusted is made by bringing afluid having a predetermined refractive index into contact with the sidewalls of the ridge portion 76 b.

Description of the method of producing the laser device 31 will becontinued.

The ridge type LD element 70 that has the configuration and is producedas described above is placed in the package 31, into which a fluidhaving a predetermined refractive index, for example silicon oil 57, islet flow in through the inlet port 32. Refractive index of the siliconoil 57 can be adjusted similarly to the method described in conjunctionwith the first embodiment. Specifically, a near field image or a farfield image of the ridge type LD element 70 is observed while changingthe mixing proportion of the silicon oil 57 thereby changing therefractive index and, when the desired image is obtained, supply of thesilicon oil 57 is stopped and the inlet aperture 32 and the outletaperture 33 are sealed with the plugs 36.

In order to make satisfactory adjustment of the photon densitydistribution in the active layer 74, for the ridge type LD element 70formed with the ridge width W of the ridge portion 76 b greater than thepredetermined width in comparison to the thickness d of the residuallayer 76 a, refractive index of the silicon oil 57 is increased so as toincrease the intensity of light leaking through the side wall of theridge portion 76 b to the outside, thereby reducing the photon densityin the active layer located right below the ridge portion 76 b. Thismakes it possible to prevent optical damage from occurring in the laserand elongate the service life of the ridge type LD element 70.

In case the ridge width W of the ridge portion 76 b is made smaller thanthe predetermined width in comparison to the thickness d of the residuallayer 76 a, refractive index of the silicon oil 57 is decreased so as toincrease the degree of light confinement in the active layer locatedright below the ridge portion 76 b. This results in increased gain ofthe laser and lower threshold of oscillation. Moreover, it is madepossible to make a laser device of higher power efficiency.

Thus according to this embodiment, a laser device having a high powerefficiency ridge type LD element of high reliability can be madeallowing it to adjust the laser characteristics after producing theridge type LD element, and therefore the production process can havegenerous margin so that a high yield can be achieved, thus reducing theproduction cost of the ridge type LD element.

C. Embodiment 3

FIG. 7 is a perspective view of the DFB-LD used in the laser deviceaccording to the third embodiment of the present invention. FIG. 6 is asectional view of the laser device of this embodiment.

In FIG. 7 and FIG. 8, reference numerals identical with those of thefirst embodiment denote the same or corresponding components.

In FIG. 7, reference numeral 80 denotes the DFB-LD. In contrast to theDFB-LD 40 of the first embodiment where the semiconductor mesa 61 isprovided to penetrate through both end faces in the light emittingdirection, this embodiment is different in that the semiconductor mesa61 is not formed in the portions of the windows 63, 64 on both ends.Thus portions near both ends where the side walls of the semiconductormesa 61 contact the fluid are formed in closed recesses 81. In otherregards, the structure is similar to that of the DFB-LD of the firstembodiment.

In FIG. 8, reference numeral 31 a denotes a base that serves as apartition wall to separate the fluid that fills the package 31. Formedon top of the base 31 a are holes 82 having edges that correspond to therecesses 81 of the DFB-LD 80. When the recesses 81 of the DFB-LD 80 andthe holes 82 of the base 31 a are aligned and the DFB-LD 80 isdie-bonded onto the base 31 a with the junction facing down, a sealedcavity 83 is formed by the base 31 a, DFB-LD 80 and the recesses 81. Thecavity 83 is filled with a fluid. The fluid, in this case silicon oilsimilarly to the first embodiment, is circulated through the inlet port32 and the outlet port 33 while changing the refractive index of thefluid 57, thereby adjusting the oscillation wavelength of the laserdevice 30. Even when the fluid is sealed in the cavity 83, the fluiddoes not enter the cavity 85 that is delimited by the outer walls 84 ofthe package 31 since the fluid is sealed by the base 31 a and the DFB-LD80. As a result, laser light emitting portion of the DFB-LD 80 does notmake contact with the fluid that enters the cavity 83 in spite of thelight emission window (not shown) being disposed to oppose the laserlight emitting portion of the DFB-LD 80.

Therefore, the outer walls 84 of the package 31 is not necessarilyrequired.

The cathode of the DFB-LD 80 and the cathode terminal 86 are connectedwith each other by the lead wire 58.

This embodiment also enables it to adjust the oscillation wavelengthsimilarly to the first embodiment, and therefore has effects similar tothose of the first embodiment. In addition, this embodiment has sucheffect as the light emission end face of the DFB-LD 40 does not contactthe silicon oil 57 so that the reflectivity of the window 64 of thelight emission end face does not change in response to the silicon oil,thus making it possible to maintain the desired emission efficiencywithout deterioration in the single-mode operation performance.

FIG. 9 is a perspective-view of a ridge type LD element used in a laserdevice according to the third embodiment of the present invention.

In this ridge type LD element 90, too, the semiconductor mesa 61 doesnot have mesa structure in the portions of windows 63, 64 near bothends. In other regards, the structure is similar to that of the ridgetype LD element 70 of the second embodiment.

Thus portions near both ends where the side walls of the semiconductormesa 61 contact the fluid are formed in closed recesses 81. In thisridge type LD element 90, too, when the recesses 81 of the ridge type LDelement 90 and the holes 82 of the base 31 a are aligned with eachother, and die-bonded onto the base 31 a with the junction facing down,the sealed cavity 83 is formed by the base 31 a, the ridge type LDelement 90 and the recesses 81 in a package 31 shown in FIG. 8. Thecavity 83 is filled with a fluid. The fluid 57, in this case silicon oil57 similarly to the second embodiment, is circulated through the inletport 32 and the outlet port 33 while changing the refractive index ofthe fluid 57, thereby adjusting the photon density in the active layerlocated right below the ridge portion 76 b of the ridge type LD element90.

This embodiment also has effects similar to those of the secondembodiment. In addition, this embodiment has such an effect as the lightemission end face of the ridge type LD element 90 does not contact thefluid 57 so that the reflectivity of the window 63 of the light emissionend face does not change in response to the fluid 57, thus making itpossible to maintain the desired emission efficiency.

Industrial Applicability

As will be understood from the foregoing description, the laser deviceaccording to the present invention is useful as the light source forwavelength division multiplexing transmission in such applications asoptical signal transmission through trunk lines such as submarine cable.

What is claimed is:
 1. A laser device comprising: a semiconductorsubstrate, a semiconductor laser element having a waveguide thatincludes a first cladding layer of a first conductivity type, an activelayer, and a second cladding layer of a second conductivity type,disposed successively on said substrate, a part of said layers having awidth that allows light to leak from side walls of said waveguide alonga direction of light propagation, a case having an aperture foradmitting a fluid and sealing, said case housing said semiconductorlaser element and surrounding said side walls of said waveguide and, afluid in said case contacting said side walls of said waveguide andhaving a refractive index.
 2. The laser device according to claim 1wherein said waveguide includes a diffraction grating layer.
 3. Thelaser device according to claim 1 wherein a part of said second claddinglayer has a ridge that has a width which allows light to leak.
 4. Thelaser device according to claim 1 wherein said case includes a firstpart having said aperture and a second part, said first part and saidsecond part being separated by a partition wall that seals said fluid,said second part having an emission window opposite light emitting endface of said laser element.
 5. The laser device according to claim 4wherein said waveguide includes a diffraction grating layer.
 6. Thelaser device according to claim 4 wherein a part of said second claddinglayer has a ridge that has a width which allows light to leak.
 7. Thelaser device according to claim 4 wherein said aperture and saidemission window are located at the walls that form a cavity.
 8. Thelaser device according to claim 7 wherein said waveguide includes adiffraction grating layer.
 9. The laser device according to claim 7wherein a part of said second cladding layer has a ridge that has awidth which allows light to leak.
 10. The laser device as in claim 1wherein said fluid is silicone oil.
 11. A method of producing asemiconductor device comprising: preparing a laser device by placing asemiconductor laser element in a case, said semiconductor laser elementhaving a semiconductor substrate and a waveguide that includes a firstcladding layer of a first conductivity type, an active layer and asecond cladding layer of a second conductivity type disposedsuccessively on said on said waveguide, a part of said layers having awidth that allows light to leak from side walls of said waveguide alonga direction of light propagation, and said case having an aperture foradmitting a fluid and sealing, said case housing said semiconductorlaser element and surrounding the side walls of said waveguide, andadjusting a characteristic of said laser element by admitting fluidshaving different refractive indices successively through said apertureinto said case while operating said semiconductor laser element andmeasuring characteristic of said laser element.
 12. The method ofproducing a semiconductor device according to claim 11 wherein preparinga laser device comprises providing an inlet aperture and an outletaperture as said aperture, and adjusting a characteristic of said laserelement by admitting fluids having different refractive indicessuccessively through said inlet aperture and extracting said fluidsthrough said outlet aperture.
 13. The method of producing asemiconductor device according to claim 11 wherein the characteristic isoscillation wavelength.
 14. The method of producing a semiconductordevice according to claim 11 wherein said characteristic is photondensity of the active layer.
 15. The method of producing a semiconductordevice according to claim 12 wherein the characteristic is oscillationwavelength.
 16. The method of producing a semiconductor device accordingto claim 12 wherein said characteristic is photon density of the activelayer.
 17. The laser device as in claim 4 wherein said fluid is siliconeoil.
 18. The laser device as in claim 7 wherein said fluid is siliconeoil.